# std::unordered_set<Key,Hash,KeyEqual,Allocator>::extract

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< cpp‎ | container‎ | unordered set

node_type extract( const_iterator position ); | (1) | (since C++17)
node_type extract( const Key& k ); | (2) | (since C++17)
template< class K > node_type extract( K&& x ); | (3) | (since C++23)

1) Unlinks the node that contains the element pointed to by position and returns a node handle that owns it.
2) If the container has an element with key equivalent to k, unlinks the node that contains that element from the container and returns a node handle that owns it. Otherwise, returns an empty node handle.
3) Same as (2). This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type, and neither `iterator` nor `const_iterator` is implicitly convertible from `K`. This assumes that such `Hash` is callable with both `K` and `Key` type, and that the `KeyEqual` is transparent, which, together, allows calling this function without constructing an instance of `Key`.

In either case, no elements are copied or moved, only the internal pointers of the container nodes are repointed .

Extracting a node invalidates only the iterators to the extracted element, and preserves the relative order of the elements that are not erased. Pointers and references to the extracted element remain valid, but cannot be used while element is owned by a node handle: they become usable if the element is inserted into a container.

### Parameters

position | - | a valid iterator into this container
k | - | a key to identify the node to be extracted
x | - | a value of any type that can be transparently compared with a key identifying the node to be extracted

### Return value

A node handle that owns the extracted element, or empty node handle in case the element is not found in (2,3).

### Exceptions

1) Throws nothing.
2,3) Any exceptions thrown by the `Hash` and `KeyEqual` object.

### Complexity

1,2,3) Average case O(1), worst case O(size()).

### Notes

extract is the only way to take a move-only object out of a set:

    std::set<move_only_type> s;
    s.emplace(...);
    move_only_type mot = std::move(s.extract(s.begin()).value());

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_associative_heterogeneous_erasure` | `202110L` | (C++23) | Heterogeneous erasure in associative containers and unordered associative containers, (3)

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <string_view>
    #include <unordered_set>

    void print(std::string_view comment, const auto& data)
    {
        std::cout << comment;
        for (auto datum : data)
            std::cout << ' ' << datum;

        std::cout << '\n';
    }

    int main()
    {
        std::unordered_set<int> cont{1, 2, 3};

        print("Start:", cont);

        // Extract node handle and change key
        auto nh = cont.extract(1);
        nh.value() = 4;

        print("After extract and before insert:", cont);

        // Insert node handle back
        cont.insert(std::move(nh));

        print("End:", cont);
    }

Possible output:

    Start: 1 2 3
    After extract and before insert: 2 3
    End: 2 3 4

### See also

merge (C++17) | splices nodes from another container (public member function)
insert | inserts elements or nodes(since C++17) (public member function)
erase | erases elements (public member function)

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